System and method for optimizing the selection of ghost channels in mitigating polarization hole burning

ABSTRACT

A system and method for optimizing the selection of ghost channels to mitigate the effects of polarization hole burning in a node of an optical communication system including identifying an optical communication channel in the node for use as a ghost channel, identifying a first set of degrees carrying the optical communication channel within the node, identifying a second set of degrees within the first set of degrees, the second set containing the degrees with the optical communication channel being a valid channel, identifying a third set of degrees within the first set of degrees, the third set containing degrees with the optical communication channel being sufficiently powerful, and selecting a first degree to source the ghost channel from the first set of degrees based at least on the second set of degrees and the third set of degrees.

TECHNICAL FIELD

This invention relates generally to the field of optical communicationnetworks and more specifically to optimizing the selection of ghostchannels in mitigating the effects of polarization hole burning.

BACKGROUND

A communication network includes paths of nodes that route packetsthrough the network. Optical amplifiers perform an important functionwithin these networks by amplifying an optical signal in order toincrease the operational length of an optical network. In someconfigurations, the efficiency of an optical communication network maybe compromised by an effect known as polarization hole burning.

Optical communication systems designed to operate over long distancesmay suffer from multiple polarization-dependent effects that reduce theoperational efficiency of the system. Polarization hole burning (PHB) isone of these phenomena. PHB may seriously reduce the performance ofrare-earth doped fiber optical amplifiers, such as an erbium doped fiberamplifier (EDFA), used to amplify signal strength within thecommunication system.

PHB occurs when a strong, polarized optical signal is launched into anEDFA. This strong signal can cause anisotropic saturation of theamplifier. This saturation effect, which is related to the populationinversion dynamics of the EDFA, depresses the gain of the EDFA for lightwith the same state of polarization (SOP) as the saturating signal.Thus, PHB causes a signal having a SOP orthogonal to the saturatingsignal to have a gain greater than that of the saturating signal.

As a result, amplified spontaneous emission (ASE) noise in the SOPorthogonal to the saturating signal may accumulate faster than in theSOP of the saturating signal. In a communication system utilizing achain of EDFAs operating at or near saturation, ASE noise may accumulateat each amplifier stage. As the noise builds up over the course of thesystem, the signal-to-noise ratio (SNR) for a signal with a SOPorthogonal to the saturating signal may rise to unacceptable levels. TheSNR in such cases can then cause errors in the received data stream.Accordingly, mitigating the effects of PHB in amplified optical systemsis desirable.

One of the causes of the undesirable PHB effect is operating an EDFA ina way that leads to gain compression. Gain compression (“Cp”) is ameasure of the difference of the amplifier's non-saturated gain (“Go,”or the gain when operating on a low power signal) and the amplifier'ssaturated operating gain (“G”). The operating gain, in decibels, can bemeasured by taking the difference between the saturated output power(“So”) and the input power of a saturating signal (“Si)”, as follows:G=So−Si.

The corresponding gain compression may be calculated as the differencebetween the non-saturated gain and the saturated operating gain:Cp=Go−G.

The gain in the SOP orthogonal to a saturating signal may be measuredusing a probe signal with an input signal orthogonal to the saturatingsignal by measuring the input power (“Pi”) and output power (“Po”) ofthe probe signal:Po−Pi=G+ΔG.

The “ΔG” in the above formula represents the amount of PHB in the SOPorthogonal to the saturation signal. This is a result of operating theamplifier with a saturating signal. As gain compression of an amplifierincreases, so does the amount and effect of PHB. For instance, a singleEDFA operating at a gain compression of about 3 dB may produce a PHB ofabout 0.08 dB. However, when that EDFA operates in a more saturatedcondition, with Cp=9-10 dB, the PHB may rise to about 0.2 dB.

The degree of PHB may also be affected by other factors, such as thedegree of polarization of the saturating signal. If a signal's SOPvaries over time, the effects of PHB may be reduced.

While the degree of PHB may be small for a single EDFA, these effectsmay be seriously compounded in communication systems that chain togethera series of EDFAs. A number of arrangements have been proposed forreducing the effects of PHB in optical communication systems. However,such arrangements continue to suffer from drawbacks such as an inabilityto deal with arbitrary channel loading, expense and difficulty ofimplementation, and the innate stability characteristics of rare-earthdoped fiber amplifiers.

SUMMARY OF THE DISCLOSURE

In accordance with the present invention, disadvantages and problemsassociated with previous techniques for mitigating the effects ofpolarization hole burning in optical amplifiers may be reduced oreliminated.

According to one embodiment of the present invention, a system foroptimizing the selection of ghost channels to mitigate the effects ofpolarization hole burning in a node of an optical communication systemis provided. The system includes a controller configured to identifyingan optical communication channel in the node for use as a ghost channel,identify a first set of degrees carrying the optical communicationchannel within the node, identify a second set of degrees within thefirst set of degrees, the second set containing the degrees with theoptical communication channel being a valid channel, identify a thirdset of degrees within the first set of degrees, the third set containingdegrees with the optical communication channel being sufficientlypowerful, and select a first degree to source the ghost channel from thefirst set of degrees based at least on the second set of degrees and thethird set of degrees.

According to another embodiment of the present invention a method foroptimizing the selection of ghost channels to mitigate the effects ofpolarization hole burning in a node of an optical communication systemis provided. The method includes identifying an optical communicationchannel in the node for use as a ghost channel, identifying a first setof degrees carrying the optical communication channel within the node,identifying a second set of degrees within the first set of degrees, thesecond set containing the degrees with the optical communication channelbeing a valid channel, identifying a third set of degrees within thefirst set of degrees, the third set containing degrees with the opticalcommunication channel being sufficiently powerful, and selecting a firstdegree to source the ghost channel from the first set of degrees basedat least on the second set of degrees and the third set of degrees.

Certain embodiments of the invention may provide one or more technicaladvantages. A technical advantage of one embodiment may be thatselecting and optimizing ghost channels from the appropriate degree of anode of a communication system increases the signal to noise ratios ofthose signals with adjacent ghost channels. In some embodiments,selecting from a set of degrees that is sufficiently valid, fresh,and/or powerful may provide a high-quality implementation forpolarization hole burning mitigation.

Certain embodiments of the invention may include none, some, or all ofthe above technical advantages. One or more other technical advantagesmay be readily apparent to one skilled in the art from the figures,descriptions, and claims included herein.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and itsfeatures and advantages, reference is now made to the followingdescription, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 illustrates an optical communication network system, inaccordance with certain embodiments of the present disclosure;

FIG. 2 illustrates a node with a plurality of degrees, in accordancewith certain embodiments of the present disclosure;

FIG. 3 illustrates a graph of a traffic channel surrounded on eitherside by ghost channels, in accordance with certain embodiments of thepresent disclosure;

FIG. 4 illustrates an optical amplification scheme for generating ghostchannels, in accordance with certain embodiments of the presentdisclosure;

FIG. 5 is a flowchart illustrating one embodiment of a method ofmitigating the effects of polarization hole burning, in accordance withcertain embodiments of the present disclosure;

FIG. 6 illustrates a table for storing information regarding thevalidity of a channel incoming to a degree of node, in accordance withcertain embodiments of the present disclosure;

FIG. 7 illustrates a table for storing information regarding thevalidity and signal strength of a channel incoming to a particulardegree of node, in accordance with certain embodiments of the presentdisclosure;

FIG. 8 illustrates a series of tables representing the consolidatedinformation received from other degrees of node, in accordance withcertain embodiments of the present disclosure;

FIG. 9 illustrates a table for storing information regarding the currentghost channel load for degrees in node, in accordance with certainembodiments of the present disclosure;

FIG. 10 is a flowchart illustrating one embodiment of a method ofmanaging the selection of ghost channels in mitigating the effects ofpolarization hole burning, in accordance with certain embodiments of thepresent disclosure;

FIG. 11 is a flowchart illustrating one embodiment of a method ofselecting ghost channels for use in mitigating the effects ofpolarization hole burning, in accordance with certain embodiments of thepresent disclosure;

FIG. 12 is a flowchart illustrating one embodiment of a method ofselecting the degree of node from which to select the appropriate ghostchannels for mitigating the effects of polarization hole burning, inaccordance with certain embodiments of the present disclosure; and

FIG. 13 is a flowchart illustrating one embodiment of a method ofoptimizing a ghost channel selection routine in order to mitigate theeffects of polarization hole burning, in accordance with certainembodiments of the present disclosure.

DETAILED DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention and its advantages are bestunderstood by referring to FIGS. 1 through 13 of the drawings, likenumerals being used for like and corresponding parts of the variousdrawings.

FIG. 1 illustrates an optical communication network system, inaccordance with certain embodiments of the present disclosure. Opticalnetwork system 10 includes components such as network nodes 22. Ingeneral, a network node 22 may include any suitable arrangement ofcomponents operable to perform the operations of the network node. As anexample, a network node may include logic, an interface, memory, othercomponent, or any suitable combination of the preceding. “Logic” mayrefer to hardware, software, other logic, or any suitable combination ofthe preceding. Certain logic may manage the operation of a device, andmay comprise, for example, a processor. “Processor” may refer to anysuitable device operable to execute instructions and manipulate data toperform operations.

“Interface” may refer to logic of a network node operable to receiveinput for the network node, send output from the network node, performsuitable processing of the input or output or both, or any combinationof the preceding, and may comprise one or more ports, conversionsoftware, or both.

“Memory” may refer to logic operable to store and facilitate retrievalof information, and may comprise Random Access Memory (RAM), Read OnlyMemory (ROM), a magnetic drive, a disk drive, a Compact Disk (CD) drive,a Digital Video Disk (DVD) drive, removable media storage, any othersuitable data storage medium, or a combination of any of the preceding.

Network system 10 communicates information through signals, such as anoptical signal. As an example, an optical signal may have a frequency ofapproximately 1550 nanometers and a data rate of 10, 20, 40, or over 40gigabits per second.

According to the illustrated embodiment, network system 10 may includeone or more networks. A network may include nodes 22 coupled by fibers26 in a mesh topology as shown in FIG. 1 or any other suitable topology,such as a liner or ring topology.

The components of network system 10, coupled together by the opticalfibers 26, may include one or more reconfigurable optical add/dropmultiplexers (ROADM), one or more amplifiers, and one or more splitters,as described in more detail below with reference to FIG. 2. Networksystem 10 may be used in any optical communication network, or any othersuitable network or combination of networks. Optical fibers 26 compriseany suitable type of fiber, such as a Single-Mode Fiber (SMF), EnhancedLarge Effective Area Fiber (E-LEAF), or TrueWave® Reduced Slope (TW-RS)fiber.

In a given topology, each node 22 will have an associated number of“degrees.” The number of degrees of node 22 may be defined to be thenumber of links incident to node 22. In the illustrated embodiment, aportion of a mesh topology consisting of four nodes 22 is shown. Eachnode 22 has four degrees: three links to the other nodes 22 of networksystem 10 and a link to the remaining portion of network system 10. Thenumber of degrees may be any number, depending on the particulartopology and implementation chosen.

In some embodiments, network system 10 may be designed to assign eachincoming signal to a particular “channel,” or carrier wavelength. Thenumber of channels and the wavelengths assigned may vary depending onthe chosen implementation. As an illustrative example, network system 10may carry 88 channels in the 1550 nm wavelength band, with a channelseparation of 50 GHz (˜0.4 nm). That is, network system 10 maypotentially communicate information on carrier wavelengths between1528.77 nm (196.1 THz) and 1563.45 nm (191.75 THz). In some embodiments,network system 10 may include some means of dynamically allocatingincoming signals to various wavelengths, depending on the design needs,such that none, some, or all channels are in use at one time.

The process of communicating information over multiple channels of asingle optical path is referred to in optics as wavelength divisionmultiplexing (WDM). Dense wavelength division multiplexing (DWDM) refersto the multiplexing of a larger (denser) number of wavelengths, usuallygreater than forty, onto a fiber. WDM, DWDM, or other multi-wavelengthtransmission techniques are employed in optical networks to increase theaggregate bandwidth per optical fiber. Without WDM or DWDM, thebandwidth in networks would be limited to the bit rate of solely onewavelength. With more bandwidth, optical networks are capable oftransmitting greater amounts of information. Referring back to FIG. 1,network system 10 is operable to transmit disparate channels using WDM,DWDM, or some other suitable multi-channel multiplexing technique, andto amplify the multiplexed wavelengths 40.

FIG. 2 illustrates a node 22 with a plurality of degrees 200, inaccordance with certain embodiments of the present disclosure. In theillustration, four degrees 200 are shown, but more or fewer degrees 200may be present in a given configuration. Each degree 200 of node 22 mayinclude splitter 202, wavelength selective switch (WSS) 204, opticalchannel monitor (OCM) 208, and one or more amplifier(s) 206. Inoperation, degree 200 of node 22 receives the multiplexed wavelengths 40from another degree 200 of node 22, another node 22, or some otherportion of network system 10. The channels comprising the multiplexedwavelengths 40 incoming to degree 200 of node 22 may be added, dropped,and/or amplified before exiting degree 200 of node 22.

Amplifier 206 may be used to amplify the multiplexed wavelengths 40.Amplifier 206 may be positioned before and/or after certain lengths offiber 26. Amplifier 206 may comprise an optical repeater that amplifiesthe optical signal. This amplification may be performed withoutopto-electrical or electro-optical conversion. In some embodiments,amplifier 206 may comprise an optical fiber doped with a rare-earthelement. When a signal passes through the fiber, external energy isapplied to excite the atoms of the doped portion of the optical fiber,which increases the intensity of the optical signal. As an example,amplifier 206 may comprise an erbium-doped fiber amplifier (EDFA).However, any other suitable amplifier 206 may be used. In theillustrated embodiment, each degree 200 of node 22 has a plurality ofamplifiers 206 on an amplifier card 212. Amplifier card 212 may, in someembodiments, also be configured to gather information about multiplexedwavelengths 40, as described in more detail below with respect to FIGS.6-10. Although the figure shows each degree 200 with its own amplifiercard 212, there may be a single amplifier card 212 for some or alldegrees 200 of node 22 or multiple nodes 22.

When the optical strength of the multiplexed wavelengths 40 reaches acertain point, amplifier 206 may reach its maximum linear response. Pastthis point, amplifier 206 may behave in a non-linear fashion (referredto as being “saturated”). At these levels, the multiplexed wavelengths40 may be referred to as a “saturating signal.” When amplifier 206 issupplied with a saturating signal, it may experience greater degrees ofcertain negative effects such as polarization hole burning (PHB). PHBmay act to create a difference in the amount of amplified spontaneousemission noise in the same SOP as the saturating signal and theamplified spontaneous emission noise in the SOP orthogonal to thesaturating signal. This difference leads to an overall decrease in thesignal-to-noise ratio. Additionally, the magnitude of the decrease isdependent on the polarization state of the saturating signal and thusvaries over time. This can result in both a reduction of signal qualityand a time-varying signal quality at later nodes 22. The effects of PHBcan be mitigated through the use of ghost channels generated from theAmplified Spontaneous Emission (ASE) noise present within network system10, as described in more detail below with reference to FIGS. 3-13.After amplification, if required, the multiplexed wavelengths 40 maythen pass to splitter 202.

Splitter 202 may include any device or component of a device which maybe configured to reproduce the multiplexed wavelengths 40 (at greater orlesser magnitudes) before passing on multiple copies of the multiplexedwavelengths 40 to other components of node 22 or to other nodes 22 ofnetwork system 10. One such copy of multiplexed wavelengths 40 may bepassed to WSS 204. WSS 204 may include any device or component of adevice which may be configured to receive, combine, add, drop, and/oramplify the component channels of the incoming multiplexed wavelengths40 transmitted from other degrees 200 of node 22 or other nodes 22 ofnetwork system 10. In some embodiments, WSS 204 may be configured togenerate ghost channels and/or amplify or attenuate previously generatedghost channels, as described in more detail below with reference toFIGS. 3-5.

OCM 208 may include any component or set of components operable toprovide information on the optical power of the individual opticalchannels comprising the multiplexed wavelengths 40. In some embodiments,OCM 208 may be an integrated part of switch card 214, such as that foundon the Fujitsu Flashwave 7500 ROADM and Flashwave 9500 ROADM. In otherembodiments, OCM 208 may be a stand-alone component, or the functions ofOCM 208 may be performed by WSS 204 or any other appropriatelyconfigured component of node 22. In operation, OCM 208 may measure atleast the optical power of the signal component channels in order toprovide this power information to other components of node 22 for theappropriate generation, selection, management, and optimization of ghostchannels for mitigating the effects of PHB. Switch card 214, or acomponent of switch card 214, may also be configured to provide thefunctions of splitter 202, WSS 204, OCM 208, or any other necessaryfunctions, such as the maintenance of information regarding the currentghost channel load of degree 200, as described in more detail below withreference to FIGS. 6-10.

FIG. 3 illustrates a graph 300 of a traffic channel 304 surrounded oneither side by ghost channels 302, in accordance with certainembodiments of the present disclosure. In the illustrated embodiment,traffic channel 304 and ghost channels 302 are shown at specificwavelengths and specific amplitudes. These and other specific propertiesof traffic channel 304 and ghost channels 302 are intended forillustrative purposes only, and are in no way intended to limit thescope of the present disclosure.

In some embodiments, node 22 comprises certain modules configured togenerate ghost channels 302 surrounding traffic channel 304. These ghostchannels 302 may operate to reduce the deleterious effects of PHB. Aghost channel 302 generally refers to the propagation of optical energythrough a communication channel of network system 10 withouttransmitting any signal information. Ghost channels 302 may be treatedlike other channels in need of amplification and/or propagation, but donot carry signal information.

Ghost channels 302 have been amplified to have an optical strength at ornear the optical strength of traffic channel 304 as described in moredetail below with reference to FIG. 4. Traffic channel 304 is a channelwithin a multi-channel optical signal that is carrying information alongits carrier wavelength at a given moment. Generation, selection,management, and optimization of the ghost channels are discussed in moredetail below with reference to FIGS. 4-13.

In some embodiments, traffic channel 304 may have only a single ghostchannel 302 on either side. In other embodiments, there may be two,four, or any number less than the current capacity of network system 10.In some configurations, the use of higher numbers of ghost channels 302may result in the possibility of feedback loops within network system10. A given implementation may balance the desire for increasing thenumber of ghost channels 302 (and the corresponding decrease in theeffects of PHB) and the desire to avoid feedback.

A given channel may have one or more “neighbor” channels. Neighborchannels are defined generally to be those that are within a certainwavelength distance of a given channel. For instance, if network 10 hasimplemented a channel separation of 50 GHz (˜0.4 nm), then channels at1544.92 nm and 1545.72 nm may be the neighbors of the channel at 1545.32nm. In other embodiments, a neighbor channel may be defined to be withinan optical bandwidth equivalent to two or more times the channel spacingbandwidth (e.g., 50 GHz in the illustrative example), depending on thesystem's design criteria (such as the fear of signal interference andthe intrinsic properties of the chosen carrier wavelength). Neighborchannels may also be said to “surround” a signal channel.

The effects of PHB may be most severe in situations where a particulartraffic channel 304 is isolated from other traffic channels 304. Ifnetwork system 10 is operating at full capacity, with all channelscarrying information at the same time, then each operating channel issurrounded by other operating channels. In such a case, the surroundingchannels function to mitigate the effects of PHB for any given channel.However, rarely does network system 10 operate at such a full capacity.Often only a percentage of the communication channels carry informationat any given time.

Some prior solutions have randomly rotated the SOP of communicationsignals so that PHB effects will not be severe in any particular stateof polarization. However, such solutions may be cost prohibitive toimplement and, depending on the chosen implementation for rotating theSOP of the saturating signal, may not be able to successfully handledynamic loading of channels across network system 10 (situations wherethe number and identity of information-carrying channels changes overtime). In such situations, PHB effects may be mitigated by surrounding acommunication channel with ghost channels.

In some embodiments, traffic channel 304 may be surrounded by ghostchannels if it does not have another traffic channel 304 sufficientlynearby. This determination may be made in accordance with a set ofpredetermined rules. For instance, if, for a first optical signal, thenext nearest optical signal is more than 20 channels away, then thefirst optical signal may require accompanying ghost channels in order tomitigate the effects of polarization hole burning. The more isolated asignal, the stronger the need for ghost channels. Below are examplesituations in which it might be desirable to produce ghost channels 302for a given traffic channel 304 (denoted by λ_(i)).if . . . λ_(i)∈{ch1, . . . , ch22} . . . and . . . λ_(i−1) &λ_(i+1)∉{ch1, . . . , ch23} and N={1, . . . , 42},

-   -   λ_(i−1), λ_(i+1) are channels adjacent to λ_(i) and N is the        total number of channels propagating in a span.        if . . . λ_(i)∈{ch23, . . . , ch44} . . . and . . . λ_(i−1) &        λ_(i+1)∉{ch22, . . . , ch44}, and N={1, . . . , 42}    -   λ_(i−1), λ_(i+1) are channels adjacent to λ_(i). and N is the        total number of channels propagating in a span.        if . . . λ_(i), λ_(i+1)∈{ch1, . . . , ch22} . . . and . . .        λ_(i−1) & λ_(i+2)∉{ch1, . . . , ch23}, and N={2, . . . , 42}    -   λ_(i−1), λ_(i+1) are channels adjacent to λ_(i) and λ_(i),        λ_(i+2) are channels adjacent to λ_(i+1). N is the total number        of channels propagating in a span.        if . . . λ_(i), λ_(i+1)∈{ch23, . . . , ch44} . . . and . . .        λ_(i−1) & λ_(i+2)∉{ch22, . . . , ch44}, and N={2, . . . , 42}    -   λ_(i−1), λ_(i+1) are channels adjacent to λ_(i) and λ_(i),        λ_(i+2) are channels adjacent to λ_(i+1). N is the total number        of channels propagating in a span.

In some embodiments, there may also be rules to determine when ghostchannels 302 may not be generated. For example, if traffic channel 304(denoted by λ_(i)) falls within the rule described below, then it maynot be sufficiently isolated to warrant the generation of ghost channels302.if . . . n( . . . , λ_(i−1), λ_(i), λ_(i+1), . . . )≧3 . . . and ( . . ., λ_(i−1), λ_(i), λ_(i+1), . . . )∈{ch1, . . . , ch44}, and N={3 . . . ,44}

-   -   . . . , λ_(i−1), λ_(i), λ_(i+1), . . . are neighbouring        channels, n is the number of neighbouring channels, and N is the        total number of channels propagating in a span.

FIG. 4 illustrates an optical amplification scheme 400 for generatingghost channels 302, in accordance with certain embodiments of thepresent disclosure. In the illustrated embodiment, traffic channel 304and ghost channels 302 are shown at specific amplitudes and separations.These and other specific properties of traffic channel 304 and ghostchannels 302 are intended for illustrative purposes only, and are in noway intended to limit the scope of the present disclosure. When trafficchannel 304 is amplified, e.g., by amplifier 206 of node 22, a certainamount of noise is introduced through a phenomenon known as spontaneousemission. The amplification of traffic channel 304 may also amplify thisnoise, resulting in amplified spontaneous emission (ASE), an undesirableand problematic noise source, particularly for long-haul systems wheretraffic channel 304 may be amplified multiple times along its path. ASEis typically absorbed or extracted from network system 10 in order tomaintain an acceptable signal to noise ratio.

However, controlling the location and magnitude of ASE may provide asource for the generation of ghost channels. Referring back to FIG. 4,four nodes 22 of network system 10 are shown, labeled as nodes 22 a, 22b, 22 c, and 22 d. These labels are intended for clarity of discussionand are in no way intended to limit the scope of this disclosure. Atraffic channel 304 may be added at node 22 a. At node 22 a, ASE at allincoming channels may be blocked, as shown in loss diagram 402 a. Powerspectrum 404 a demonstrates that, as traffic channel 304 is transmittedfrom node 22 a to node 22 b, only traffic channel 304 is transmitted.Loss diagrams 402 and power spectrums 404 are provided for illustrativepurposes only. As an example, in some embodiments, there may be multipletraffic channels 304 at varying separations.

When traffic channel 304 is amplified at node 22 b, the ASE in thechannels neighboring traffic channel 304 are not absorbed or extracted.The ASE in these neighboring channels may be allowed to grow to acertain point in order to provide the appropriate ghost channels 302surrounding traffic channel 304. Loss diagram 402 b shows that the blocklevels applied to the neighbor channels have been reduced to asubstantially lower level. This may allow the ASE in the neighborchannels to grow, generating a ghost channel, as shown in power spectrumdiagram 404 b.

At this stage, ghost channels 302 may have a low optical strengthrelative to traffic channel 304. As traffic channel 304 and ghostchannels 302 are passed through a third node 22 c, ghost channels 302may be amplified to a greater optical strength. Loss diagram 402 c showsthat the block levels applied to the neighbor channels are at the samesubstantially lower level as those in loss diagram 402 b. This may allowthe ASE in the neighbor channels to continue to grow, allowing ghostchannels 302 to gain optical power, as shown in power spectrum diagram404 c.

This amplification may continue through a fourth node 22 d until theoptical strength of ghost channels 302 is at or near the opticalstrength of traffic channel 304. At this stage ghost channels 302 may beat their most efficient in mitigating the effects of polarization holeburning without overcoming traffic channel 304. Loss diagram 402 d showsthat the block levels applied to the neighbor channels have been raisedrelative to loss diagram 402 c, but are still at a substantially lowerlevel than in loss diagram 402 a. This may allow the ASE in the neighborchannels to be capped at a certain optical power level, allowing ghostchannels 302 to propagate at an optical power level substantially equalto traffic channel 304, as shown in power spectrum diagram 404 d.

Although this figure depicts this process taking place over the courseof four nodes 22 a-D, a particular implementation may take more or fewernodes to get ghost channels 302 to an appropriate optical strength.

In some embodiments, degree 200 of node 22 may amplify ghost channels302. As part of the amplification, degree 200 may also determine whetherghost channel 302 needs to be amplified, or if it is already ofsufficient magnitude. For instance, in some embodiments, it may bedesirable to cap amplification of a ghost channel 302 as described inmore detail below with reference to FIGS. 5-13. In other embodiments,the amplification and measurement may be done by more or differentcomponents of network system 10.

FIG. 5 is a flowchart illustrating one embodiment of a method 500 ofmitigating the effects of polarization hole burning, in accordance withcertain embodiments of the present disclosure. Method 500 includeschecking an incoming traffic channel 304 for the existence ofneighboring ghost channels 302, generating ghost channels 302 ifnecessary, and amplifying ghost channels 302 if necessary.

According to one embodiment, method 500 preferably begins at step 502.Teachings of the present disclosure may be implemented in a variety ofconfigurations of nodes 22 and network system 10. As such, the preferredinitialization point for method 500 and the order of steps 502-512comprising method 500 may depend on the implementation chosen. Asdescribed in more detail above with reference to FIG. 1, node 22 may beassociated with a number of degrees 200, each receiving multiplexedwavelengths 40 from a different portion of network system 10. Dependingon the implementation chosen, method 500 may be performed on some, all,or none of degrees 200 of node 22. Additionally, multiplexed wavelengths40 may be a multi-channel signal which may be demultiplexed into itscomponent channels. Depending on the implementation chosen, method 500may be performed on some, all, or none of the traffic channels 304 ofmultiplexed wavelengths 40.

At step 502, degree 200 of node 22 receives multiplexed wavelengths 40.After receiving multiplexed wavelengths 40, method 500 may begin toanalyze a first channel constituting multiplexed wavelengths 40. Afteranalyzing that channel, method 500 may proceed to step 503, where node22 may determine whether the channel under consideration is a trafficchannel 304. In some embodiments, determining whether a given channel isa traffic channel 304 may constitute examining the WCS and WCF bits forthat channel, as described in more detail below with reference to FIGS.6-10.

If the channel is not a traffic channel 304, then method 500 may proceedto step 512, where method 500 may proceed to examine the next channelbefore returning to step 502. If the channel under consideration is atraffic channel 304, then method 500 may proceed to step 504, where node22 may determine whether there are existing ghost channels 302 fortraffic channel 304. In some embodiments, step 504 may be performed bysoftware controlling wavelength selection switch 204 of node 22, or anyother appropriately configured measurement module, as described in moredetail above with reference to FIGS. 1-4. In other embodiments, step 504may be performed by hardware, firmware, or any other software module,including the operating system controlling node 22 configured todetermine the presence of ghost channels 302.

If no ghost channels 302 are currently present, method 500 may proceedto step 506, where node 22 may generate ghost channels 302. In someembodiments, step 506 may be performed by wavelength selection switch204 of node 22, or any other ghost channel generation module configuredto modify the blocking level of ASE in appropriate channels, asdescribed in more detail above with reference to FIGS. 1-4. Aftergenerating ghost channels 302, method 500 may proceed to step 508. If,in step 504, method 500 determined that there were extant ghost channels302, method 500 may proceed directly to step 508.

At step 508, method 500 may determine the power of ghost channels 302,as measured by optical channel monitor 208 or any other appropriatelyconfigured power monitor, as described in more detail above withreference to FIGS. 1-4. Method 500 may then compare the power of ghostchannels 302 to the power of the associated traffic channel 304. In someembodiments, this comparison may be performed by wavelength selectionswitch 204 of node 22, or any other appropriately configured comparator,as described in more detail above with reference to FIGS. 1-4. If ghostchannels 302 are not of a sufficient magnitude, then method 500 mayproceed to step 510, where ghost channels 302 are amplified, asdescribed in more detail above with reference to FIGS. 1-4. Afteramplification, method 500 may proceed to step 512.

If, in step 508, method 500 determined that extant ghost channels 302were already of a sufficient magnitude, then method 500 may proceeddirectly to step 512. At step 512, method 500 may proceed to examine thenext channel before returning to step 502.

Although FIG. 5 discloses a particular number of steps to be taken withrespect to method 500, method 500 may be executed with more or fewersteps than those depicted in FIG. 5. In addition, although FIG. 5discloses a certain order of steps comprising method 500, the stepscomprising method 500 may be completed in any suitable order. Forexample, in the embodiment of method 500 shown, node 22 determineswhether ghost channels 302 are substantially equal in magnitude totraffic channel 304. However, in short-distance systems where designconsiderations may not require concern over potential overgrowth ofghost channels, these steps may not be necessary. Additionally, method500 may also include additional steps concerning the determination ofhow close a ghost channel 302 may be in order to decide if additionalghost channels 302 may be necessary.

Certain embodiments of the invention may provide one or more technicaladvantages. A technical advantage of one embodiment may be that usingghost channels 302 to mitigate the effects of polarization hole burningallows for a more robust solution to PHB effects in dynamically loadedoptical communication network systems 10. Another advantage may be that,since the methods and systems disclosed herein may be implemented inpre-existing hardware and/or software, the implementation costs anddifficulties may be substantially reduced.

Effectively mitigating the effects of polarization hole burning in anoptical communication system may require systems and/or methods ofeffectively managing, selecting, and/or optimizing the generated ghostchannels 302. Referring again to FIG. 2, in order to effectively manageghost channel generation, it may be desirable to have the degrees 200 ofnode 22 communicate with one another. Intranodal communication may beused for many purposes, including the direction of incoming traffic overdifferent output paths. Intranodal communication may also be used forcommunicating the current state of particular channels passing througheach degree 200 of node 22. This information can be important in themanagement of ghost channels 302 used for polarization hole burningmitigation.

In some embodiments, a degree 200 may need to add a ghost channel 302,or pass-through (and potentially amplify) an existing ghost channel 302from another degree 200 of node 22. In order to effectively balance theload of the ghost channels across node 22, it may be important to knowthe current and potential sources for ghost channels. However, theinformation required to effectively manage the ghost channel sources maynot be readily available at each degree 200 of node 22. For instance,the splitter input power for a given channel at Degree 1 may not beavailable to WSS 204 of Degree 4. Without this information, Degree 4 maybe unable to correctly determine whether to source a ghost channel fromthat given channel. In order to overcome these obstacles, degrees 200may share information.

In some embodiments, degree 200 may collect certain pieces ofinformation regarding the channels incoming to that degree 200. Thatinformation may include a wavelength channel signal bit (“WCS”), awavelength channel failure indicator (“WCF”), and the splitter inputpower for each channel. The WCS and WCF bits may be collected byamplifier card 212 of degree 200 and splitter input power may becollected by OCM 208 of degree 200. However, these functions may beperformed by the same component, different components, or anyappropriately configured channel information module.

The WCS bit may be used to indicate whether a particular wavelength isintended to be present, e.g., whether information is being sent throughthat channel. In some embodiments, a “1” may indicate that a wavelengthis intended to be present and a “0” may indicate that a wavelength isnot intended to be present. The WCF bit may be used to indicate whethera wavelength is actually present. Although a wavelength may be intendedto be present, some failure may have occurred. In some embodiments, a“1” may indicate that actual light is present, while a “0” may indicatethat no light is present. These indicators, or others like them, maywork together to indicate that a particular channel is “valid” for thepurposes of ghost channel sourcing. In the illustrated configuration, avalid channel is one that is both intended to carry information andactually carrying information. However, other configuration may be basedon different design decisions and define a valid channel differentlywithout departing from the scope of this disclosure.

In some embodiments, each degree 200 may assemble a table for thedesired channel validity information, with an entry for each incomingchannel. These tables are discussed below in further detail withreference to FIGS. 5-9. Each degree 200 may then transmit these tablesto every other degree 200 of node 22 in order to maximize informationsharing and subsequent decision making.

FIG. 6 illustrates a table 600 for storing information regarding thevalidity of a channel incoming to a degree 200 of node 22, in accordancewith certain embodiments of the present disclosure. Table 600 maycomprise a plurality of entries 602, with each entry 602 correspondingto an incoming channel. For each entry 602 table 600 may store one ormore value(s) 604. In some embodiments, values 604 are the WCS and WCFvalues for each incoming channel. WCS and WCF values are discussed inmore detail above with reference to FIG. 3. As an illustrative example,entry 602 for channel 1 may have a value of “1” for WCS and “0” for WCF.This may indicate that channel 1 is intended to be operational, but thatno light is present. Entry 602 for channel 2 may have a “1” for WCS,indicating that it is intended to be operational, and a “1” for WCF,indicating that light is, in fact, present. The channel validityinformation of table 600 may then be passed on to a portion of degree200 configured to measure optical strength of incoming channels. In someembodiments, the gathering of data in table 600 is performed byamplifier card 212 within degree 200 of node 22. Amplifier card 212 maythen send table 600 to switch card 214 in order to gather informationregarding the optical power of the incoming channels as described inmore detail below with reference to FIGS. 7-9. However, in otherembodiments, table 600 may be generated within switch card 214 of degree200 or by any channel information module of node 22 configured to gatherthe appropriate channel validity information.

FIG. 7 illustrates a table 700 for storing information regarding thevalidity and signal strength of a channel incoming to a particulardegree 200 of node 22, in accordance with certain embodiments of thepresent disclosure. Table 700 may comprise a plurality of entries 702,with each entry 702 corresponding to an incoming channel. For each entry702, table 700 may store one or more value(s) 704. In some embodiments,values 704 are the WCS, WCF, and splitter input power for each incomingchannel. WCS and WCF values are discussed in more detail above withreference to FIGS. 2 and 6. OCM 208 of degree 200 may measure thesplitter input power for each channel and record the information intable 700. Table 700 may then be broadcast to every other degree 200 ofnode 22.

In some embodiments, switch card 214 of degree 200 receives channelvalidity information from amplifier card 212 (or some other channelinformation module), as described in more detail above with reference toFIG. 6 (e.g., in the form of table 600) and appends to that informationregarding the optical power of the incoming channels from OCM 208 (e.g.,in the form of splitter input power in table 700). However, in otherembodiments, these functions may be performed by the same component. Forinstance, the channel information module and switch card 214 may be anintegral component configured to determine WCS bits, WCF bits, and tomeasure splitter input power at the same time without the need forseparate tables 600 and 700.

Once the information illustrated in table 700 has been broadcast toother degrees 200 of node 22, it may be necessary or efficient for eachdegree 200 to consolidate the tables 700 received from each of the otherdegrees 200.

FIG. 8 illustrates a series of tables 800 representing the consolidatedinformation received from other degrees 200 of node 22, in accordancewith certain embodiments of the present disclosure. In some embodiments,a degree 200 of node 22 may receive tables from other degrees 200 ofnode 22 containing information regarding the validity and optical powerof certain channels, as described in more detail above with reference toFIGS. 6-7. Aggregating such data in a way such that the data isavailable for each channel may be desirable, such as is depicted intable(s) 800. It may also be desirable to include “freshness” data toindicate how recently the data has been received.

Each table 800 may represent the aggregated information for a particularchannel. In some embodiments, there is a table 800 corresponding to eachincoming channel (as an example only, the illustrated embodimentincludes eighty-eight channels, so there are eighty-eight tables 800).Each table 800 may include a plurality of entries 802, with each entry802 corresponding to a degree 200 in node 22. Some configurations mayalso find it more desirable or efficient to combine one or more table(s)800 into a single table 800, or to split a single table 800 into smallertables.

For each entry 802 table 800 may store one or more value(s) 804. In someembodiments, values 804 are the WCS bit, WCF bit, splitter input power,and freshness values for each degree. WCS, WCF, and splitter input powervalues are described in more detail above with reference to FIGS. 6-7.The “Fresh” value of table 800 may be used to indicate whether theremaining values 804 of table 800 have been recently updated (orsufficiently “fresh”). In some embodiments, switch card 214 of degree200 may determine whether the data stored in table 800 has beenrefreshed within a predetermined amount of time, e.g., five seconds. Ifthe data has been received within that time period, then the Fresh value804 may be marked with a “1” to indicate that the data is sufficientlyfresh; if is has not, then it may be marked with a “0” to indicate thatthe data is stale.

In addition to information regarding the validity and freshness of theincoming channels at each degree 200 of node 22, it may also bedesirable to know what, if any, ghost channels are already sourced froma degree 200.

FIG. 9 illustrates a table 900 for storing information regarding thecurrent ghost channel load for degrees 200 in node 22, in accordancewith certain embodiments of the present disclosure. Table 900 mayinclude a plurality of entries 902, with each entry 902 corresponding toa degree 200 in node 22. For each entry 902 table 900 may store one ormore value(s) 904. In some embodiments, value 904 is the current ghostchannel count for each degree 200 in node 22. The ghost channel count isthe number of ghost channels that are currently sourced from aparticular degree. The data in current ghost count 904 may be used toefficiently balance the ghost channel load within node 22. Thisinformation may allow the distribution of ghost channels across degrees200 within node 22, which may in turn improve operational efficiency ofnode 22 and network system 10. Table 900 may, in some embodiments, bestored and managed by the software code that operates WSS 204 of degree200. In other embodiments, table 900 may be managed by an overarchingnode management system, and implemented in software, hardware, orfirmware, or some combination thereof. The information in table 900regarding the current ghost channel load of degrees 200 may be combinedwith the channel input information in table 600 to better manage theghost channels used to mitigate the effects of polarization holeburning.

FIG. 10 is a flowchart illustrating one embodiment of a method 1000 ofmanaging the selection of ghost channels in mitigating the effects ofpolarization hole burning, in accordance with certain embodiments of thepresent disclosure. Method 1000 includes collecting validity and opticalpower data for optical communication channels, transmitting that data toall degrees 200 within a node 22, aggregating received data, andcollecting data concerning the current ghost channel loading.

According to one embodiment, method 1000 preferably begins at step 1002.Teachings of the present disclosure may be implemented in a variety ofconfigurations of node 22 and network system 10. As such, the preferredinitialization point for method 1000 and the order of steps 1002-1010comprising method 1000 may depend on the implementation chosen.

At step 1002, method 1000 collects validity data for an opticalcommunication channel at a first degree 200 (as an example only, the WCSand WCF bits for the optical communication channel). In someembodiments, this step may be performed by a channel information moduleas described in more detail above with reference to FIGS. 6-9. Aftercollecting this information method 1000 may proceed to step 1004.

At step 1004, method 1000 collects optical power data for the opticalcommunication channel at each degree 200. In some embodiments, step 1004may be performed by OCM 208 as described in more detail above withreference to FIGS. 6-9. After collecting this information method 1000may proceed to step 1006.

At step 1006, method 1000 may transmit all of the collected validity andoptical power data to all other degrees 200 within node 22. Step 1006may be performed by switch card 214 as described in more detail abovewith reference to FIGS. 6-9. After transmitting this data, method 1000may proceed to step 10101008.

At step 1008, method 1000 may aggregate the channel validity and opticalpower data received at each degree 200 from all other degrees 200 suchthat a composite picture of the data for a given channel may be formed.The aggregated validity and power data may be combined with a freshnessvalue, indicating how recently the data had been retrieved. Step 1008may be performed by switch card 214 as described in more detail abovewith reference to FIGS. 6-9. After aggregating and collecting this data,method 1000 may proceed to step 1010.

At step 1010, method 1000 may collect data regarding the current ghostchannel load within node 22. This data may comprise informationdetailing which degrees 200 within node 22 are currently being used asghost channel sources, as described in more detail above with referenceto FIG. 9. After collecting this data, method 1000 may return to step1002 to begin the data collection cycle again. In some embodiments,there may be a time delay, such as five seconds, before the cycle beginsagain. Such a time delay would be a design determination to suit theparticular implementation of network system 10.

Although FIG. 10 discloses a particular number of steps to be taken withrespect to method 1000, method 1000 may be executed with more or fewersteps than those depicted in FIG. 10. In addition, although FIG. 10discloses a certain order of steps comprising method 1000, the stepscomprising method 1000 may be completed in any suitable order. Forexample, in the embodiment of method 1000 shown, degree 200 of node 22collects channel validity data and optical power data in two separatesteps. In some embodiments, it may be desirable to separate these stepsto have separate components of degree 200 perform these tasks. However,in other embodiments these steps may be performed simultaneously and/orby the same component of degree 200.

Certain embodiments of the invention may provide one or more technicaladvantages. A technical advantage of one embodiment may be thateffective management of ghost channel selection can allow a networksystem to effectively balance the load that ghost channels may place onthe system.

Along with managing the load balance of ghost channels 302 within node22, it may also be beneficial to select the appropriate source for ghostchannels 302 in order to reduce or eliminate the potential deleteriouseffects of feedback when mitigating the effects of polarization holeburning.

FIG. 11 is a flowchart illustrating one embodiment of a method 1100 ofselecting ghost channels for use in mitigating the effects ofpolarization hole burning, in accordance with certain embodiments of thepresent disclosure. Method 1100 includes checking each channel to see ifit is a traffic channel, a blocked channel, a current ghost channel, orif its neighbor channels are add or pass-through channels. In operation,method 1100 determines whether, for a given channel, either neighborchannel will have a signal when it leaves degree 200. If it does, thenthe current channel may need to be blocked from use as a ghost channelin order to prevent undesirable feedback within network system 10. Onepoint at which it may be effective to reduce feedback is at the pointwhere a traffic channel is added. If the traffic channel is being addedat node 22, then ghost channel 302 may not be immediately introduced atnode 22. If a traffic channel is a pass through channel, then ghostchannel 302 may be generated with less concern for feedback.

According to one embodiment, method 1100 preferably begins at step 1102.Teachings of the present disclosure may be implemented in a variety ofconfigurations of communication system 10. As such, the preferredinitialization point for method 1100 and the order of steps 1102-1116comprising method 1100 may depend on the implementation chosen. In someembodiments, the steps of method 1100 may be performed by the softwarethat manages WSS 204 of each degree 200. In other embodiments differentsteps may be performed by different pieces of software or differentsoftware modules within one piece of software, or may be implemented inhardware or firmware or any appropriate combination thereof configuredto perform the method within each degree 200.

Beginning with a first channel, at step 1102, method 1100 determineswhether the first channel is a traffic channel. In some embodiments,determining whether a given channel is a traffic channel 304 mayconstitute examining the WCS and WCF bits for that channel, as describedin more detail above with reference to FIGS. 6-10. If the channel is atraffic channel, then method 1100 may proceed to step 1108, whereinmethod 1100 advances to the next channel before returning to step 1102.If the channel is not a traffic channel, then method 1100 may proceed tostep 1104.

At step 1104, method 1100 determines whether either channel neighboringthe channel under consideration is an add channel. In some embodiments,an add channel is a channel on which traffic is added at WSS 204 of thedegree 200 performing the method. In such a situation, sourcing a ghostchannel may lead to undesirable feedback within network system 10. Ifeither neighbor channel is an add channel, then method 1100 may proceedto step 1106. If neither neighbor channel is an add channel, then method1100 may proceed to step 1112.

At step 1106 method 1100 may determine whether the channel underconsideration is currently blocked. If it is blocked, then method 1100may proceed to step 1108, wherein method 1100 advances to the nextchannel before returning to step 1102. If the current channel underconsideration is not blocked, then method 1100 may proceed to step 1110,wherein the channel is blocked, before proceeding to step 1108.

At step 1112, method 1100 may determine whether either neighbor channelis a pass-through channel. In some embodiments, a pass-through channelis a channel on which traffic is received at the degree 200 and which ispassed through WSS 204 of that degree 200. In the case of pass-throughchannels, sourcing a ghost channel from the channel under considerationmay not run as high a risk of undesirable feedback within network system10. If either neighbor channel is a pass-through channel, then method1100 may proceed to step 1114. If neither neighbor channel is apass-through channel, then method 1100 may proceed to step 1106.

At step 1114, method 1100 may determine whether the current channel isalready being used to source a ghost channel. If it is, then method 1100may proceed to step 1108, wherein method 1100 advances to the nextchannel before returning to step 1102. If it is not, then method 1100may proceed to step 1116. At step 1116, method 1100 may select thecurrent channel to source a ghost channel. The selection method may besimple or complex, depending on the particular implementation. In someembodiments, the ghost source may be selected from a calculated set ofdegrees with appropriate validity and optical power criteria, asdescribed below in more detail with reference to FIG. 12.

Although FIG. 11 discloses a particular number of steps to be taken withrespect to method 1100, method 1100 may be executed with more or fewersteps than those depicted in FIG. 11. In addition, although FIG. 11discloses a certain order of steps comprising method 1100, the stepscomprising method 1100 may be completed in any suitable order. Forexample, in the embodiment of method 1100 shown, channels with neighborsthat will carry add channels after leaving degree 200 may be blocked.However, in some configurations it may be less important to guardagainst feedback and these steps may be reduced in scope or eliminated.

FIG. 12 is a flowchart illustrating one embodiment of a method 1200 ofselecting the degree 200 of node 22 from which to select the appropriateghost channels for mitigating the effects of polarization hole burning,in accordance with certain embodiments of the present disclosure. Method1200 includes selecting the source degree 200 for the ghost channel 302identified above with reference to FIG. 11 by examining the validity andoptical power values of channels within the degrees 200 of node 22, inorder to maintain a desired level of load balancing across networksystem 10.

In operation, method 1200 may select the degree 200 of node 22 that cansource a ghost channel 302 with the highest initial optical power (e.g.,the ASE noise at the highest initial level) and/or the degree with thelowest current ghost channel count.

According to one embodiment, method 1200 preferably begins at step 1202.Teachings of the present disclosure may be implemented in a variety ofconfigurations of communication system 10. As such, the preferredinitialization point for method 1200 and the order of steps 1202-1220comprising method 1200 may depend on the implementation chosen. In someembodiments, the steps of method 1200 may be performed by the softwarethat manages WSS 204 of degree 200. In other embodiments different stepsmay be performed by different pieces of software or different softwaremodules within one piece of software, or may be implemented in hardwareor firmware or any appropriate combination thereof configured to performthe method within degree 200.

At step 1202, method 1200 may determine whether a given channel isvalid, fresh, and has a splitter input power greater than or equal to apredetermined threshold. Channel validity, freshness, and optical powerinformation are discussed in more detail above with reference to FIGS.6-10. The predetermined threshold may be any value determined by theparticular implementation of network system 10 to produce a likelihoodof finding an appropriate ghost channel while maintaining the balance ofghost channel load across degrees 200 of node 22. The set of degrees 200meeting both criteria is depicted in method 1200 by the letter “D.”After examining each degree 200, the method may proceed to step 1204.

At step 1204, method 1200 may determine whether there are any degrees200 within the set that met both criteria in step 1202 (e.g., whetherD=0). If there are any sufficiently valid, fresh, and powerful channelsin any degree 200 (D does not equal 0), then method 1200 may proceed tostep 1206. If there are not, then method 1200 may proceed to step 1212.

At step 1206, method 1200 may determine which degree currently has theleast number of ghost channels currently sourced. The gathering,collecting, and transmission of current ghost channel load informationis described above in more detail with reference to FIGS. 6-10. In FIG.12, this degree is denoted by the letter “d.” After determining theappropriate degree, method 1200 may proceed to step 1208. At step 1208,the controller may switch in the designated ghost channel from thedesignated degree d. Method 1200 may then proceed to step 1210, wherethe ghost channel count for the designated degree is incremented, atwhich point method 1200 may proceed to step 1220, where method 1200terminates.

If there are no sufficiently valid, fresh, powerful channels found atstep 1204 (e.g., D=0), then method 1200 may proceed to step 1212, wheremethod 1200 may limit the determination to the number of sufficientlyvalid and fresh channels (denoted by the letter “D.”). After determiningthe number of valid and fresh channels, method 1200 may proceed to step1214. At step 1214, the controller may determine if there were any validand fresh channels found in step 1212. If there were no valid and freshchannels (D=0), then method 1200 may proceed to step 1216, where thechannel under examination is blocked as a ghost source. After blockingthe channel, method 1200 may proceed to step 1220, where method 1200terminates.

If, in step 1214, there were one or more valid and fresh channels, thenmethod 1200 may proceed to step 1218. At step 1218, method 1200 maydetermine which degree has the channel with the highest splitter inputpower. This degree is denoted by the letter “d” in the drawing. Aftermaking this determination, method 1200 may proceed to step 1208. At step1208, method 1200 may switch in the ghost channel from the designateddegree. As discussed above, after switching in the ghost channel, method1200 may then proceed to step 1210, where the ghost count is incrementedfor the designated degree, and then to step 1220, where method 1200terminates.

Although FIG. 12 discloses a particular number of steps to be taken withrespect to method 1200, method 1200 may be executed with more or fewersteps than those depicted in FIG. 12. In addition, although FIG. 12discloses a certain order of steps comprising method 1200, the stepscomprising method 1200 may be completed in any suitable order. Forexample, in the embodiment of method 1200 shown, a choice may be made toswitch in a channel with a splitter input of less than the full ghostthreshold. However, in some configurations it may be desirable to onlysource ghost channels from channels with a splitter input greater thanor equal to the full ghost threshold.

In order to maintain the effectiveness of a load balanced, ghost channelpolarization hole burning mitigation scheme, it may be necessary ordesirable to optimize that scheme for continued performance.

FIG. 13 is a flowchart illustrating one embodiment of a method 1300 ofoptimizing a ghost channel selection routine in order to mitigate theeffects of polarization hole burning, in accordance with certainembodiments of the present disclosure. Method 1300 includes periodicallyrunning a ghost channel selection algorithm in order to ensure that themost effective degree 200 of node 22 is the current source for a ghostchannel 302.

According to one embodiment, method 1300 preferably begins at step 1302.Teachings of the present disclosure may be implemented in a variety ofconfigurations of communication system 10. As such, the preferredinitialization point for method 1300 and the order of steps 1302-1308comprising method 1300 may depend on the implementation chosen. In someembodiments, the steps of method 1300 may be performed by the softwarethat manages WSS 204 of degree 200. In other embodiments different stepsmay be performed by different pieces of software or different softwaremodules within one piece of software, or may be implemented in hardwareor firmware or any appropriate combination thereof configured to performthe method within degree 200.

At step 1302, method 1300 may run a ghost selection routine for a firstchannel. The ghost selection routine may be simple or complex, dependingon the configuration of network system 10. In some embodiments, theghost source may be selected from a calculated set of degrees withappropriate validity and optical power criteria, as described in moredetail above with reference to FIGS. 11-12. Once the ghost selectionroutine has run for the first channel, method 1300 may proceed to step1304.

At step 1304, method 1300 may determine the current source degree forthe ghost channel. Method 1300 may then proceed to step 1305. At step1305, method 1300 may determine whether the ghost channel source degreereturned from the selection routine is different from the current ghostchannel source degree. If it is not, then method 1300 may proceed tostep 1308, wherein method 1300 may proceed to the next ghost channel andrepeat the method by returning to step 1302. If the ghost channelselection routine returns a different source than what is currentlyused, then method 1300 may proceed to step 1306. At step 1306, method1300 may switch the source of the current ghost channel from the degree200 currently being used to the degree 200 returned from the ghostchannel selection routine in step 1302. After switching the ghostchannel source, method 1300 may proceed to step 1308, wherein method1300 may proceed to the next ghost channel and repeat the method byreturning to step 1302.

Although FIG. 13 discloses a particular number of steps to be taken withrespect to method 1300, method 1300 may be executed with more or fewersteps than those depicted in FIG. 13. In addition, although FIG. 13discloses a certain order of steps comprising method 1300, the stepscomprising method 1300 may be completed in any suitable order. Forexample, in the embodiment of method 1300 shown, the ghost channelselection routine is run for every ghost channel. However, in someconfigurations it may not be desirable or efficient to run the routinefor every ghost channel continuously. It may be more efficient to, forinstance, only run the routine for every other ghost channel the firsttime through method 1300, and run the routine for the other half ofghost channels the second time through method 1300.

While this disclosure has been described in terms of certain embodimentsand generally associated methods, alterations and permutations of theembodiments and methods will be apparent to those skilled in the art.Accordingly, the above description of example embodiments does notconstrain this disclosure. Other changes, substitutions, and alterationsare also possible without departing from the spirit and scope of thisdisclosure, as defined by the following claims.

1. A computerized method for selecting ghost channels in a first node ofan optical communication system, comprising: identifying an opticalcommunication channel in the node for use as a ghost channel;identifying a first set of degrees carrying the optical communicationchannel within the node; identifying a second set of degrees within thefirst set of degrees, the second set containing the degrees with theoptical communication channel being a valid channel; identifying a thirdset of degrees within the first set of degrees, the third set containingdegrees with the optical communication channel being sufficientlypowerful; selecting a first degree to source the ghost channel from thefirst set of degrees based at least on the second set of degrees and thethird set of degrees; sourcing the ghost channel at the first degree;and transmitting the ghost channel to a second node of the opticalcommunication system.
 2. The computerized method of claim 1, furthercomprising performing the above steps for each of a plurality of opticalcommunication channels in the node.
 3. The computerized method of claim1, further comprising blocking the optical communication channel in thenode for use as the ghost channel if the second set of degrees is empty.4. The computerized method of claim 1, wherein identifying the third setof degrees comprises identifying a set of degrees wherein the opticalcommunication channel has optical power greater than or equal to theoptical power of the optical communication channel in all other degreeswithin the first set of degrees.
 5. The computerized method of claim 1,wherein identifying the third set of degrees comprises identifying a setof degrees wherein the optical communication channel has optical powergreater than or equal to the optical power of the optical communicationchannel in all other degrees within the second set of degrees.
 6. Thecomputerized method of claim 1, wherein identifying the third set ofdegrees comprises identifying a fourth set of degrees within the thirdset of degrees, the fourth set of degrees containing the degrees withthe optical communication channel having optical power greater than apredetermined ghost threshold level.
 7. The computerized method of claim1, further comprising identifying a current ghost count from the firstset of degrees and the second set of degrees.
 8. The computerized methodof claim 7, further comprising incrementing a ghost count for the degreeto source the ghost channel.
 9. The computerized method of claim 8,wherein selecting the degree to source the ghost channel comprisesselecting the degree with the smallest value of the current ghost count.10. The computerized method of claim 1, further comprising: repeating,after a predetermined time period, the first four steps to select asecond degree to source the ghost channel from the first set of degreesbased at least on the second set of degrees and the third set ofdegrees; and if the second degree is different from the first degree,sourcing the ghost channel from the second degree.
 11. The computerizedmethod of claim 10, wherein the predetermined time period is a frequencydesigned to minimize system churn.
 12. The computerized method of claim11, wherein the frequency designed to minimize system churn is definedto be once per a control cycle of the optical communication system. 13.A system for selecting ghost channels in a node of an opticalcommunication system, the system comprising a controller, the controllercomprising a instructions embodied in a non-transitory computer-readablemedium, the instructions configured to: identify an opticalcommunication channel in the node for use as a ghost channel; identify afirst set of degrees carrying the optical communication channel withinthe node; identify a second set of degrees within the first set ofdegrees, the second set containing the degrees with the opticalcommunication channel being a valid channel; identify a third set ofdegrees within the first set of degrees, the third set containingdegrees with the optical communication channel being sufficientlypowerful; and select a first degree to source the ghost channel from thefirst set of degrees based at least on the second set of degrees and thethird set of degrees.
 14. The system of claim 13, wherein the controlleris a part of a wavelength selection switch.
 15. The system of claim 13,wherein the controller is a part of a switch card.
 16. The system ofclaim 13, wherein the instructions are further configured to perform theabove steps for each of a plurality of optical communication channels inthe node.
 17. The system of claim 13, wherein the instructions arefurther configured to perform the above steps for a plurality of opticalcommunication channels in the node.
 18. The system of claim 13, whereinthe instructions are further configured to block the opticalcommunication channel in the node for use as the ghost channel if thesecond set of degrees is empty.
 19. The system of claim 13, wherein theinstructions are further configured to identify the third set of degreesby identifying a set of degrees wherein the optical communicationchannel has optical power greater than or equal to the optical power ofthe optical communication channel in all other degrees within the firstset of degrees.
 20. The system of claim 13, wherein the instructions arefurther configured to identify the third set of degrees by identifying aset of degrees wherein the optical communication channel has opticalpower greater than or equal to the optical power of the opticalcommunication channel in all other degrees within the second set ofdegrees.
 21. The system of claim 13, wherein the instructions arefurther configured to identify a fourth set of degrees within the thirdset of degrees, the fourth set of degrees containing the degrees withthe optical communication channel having optical power greater than apredetermined ghost threshold level.
 22. The system of claim 21, whereinthe instructions are further configured to identify a current ghostcount from the first set of degrees and the second set of degrees. 23.The system of claim 22, wherein the instructions are further configuredto increment a ghost count for the degree to source the ghost channel.24. The system of claim 23, wherein the instructions are furtherconfigured to select the degree to source the ghost channel by selectingthe degree with the smallest value of the current ghost count.
 25. Thesystem of claim 13, wherein the instructions are further configured to:repeat, after a predetermined time period, the first four steps toselect a second degree to source the ghost channel from the first set ofdegrees based at least on the second set of degrees and the third set ofdegrees; and if the second degree is different from the first degree,source the ghost channel from the second degree.